Field of the Invention
The invention relates to a method of controlling an ink layer on a printing form of a printing machine and, thereby, controlling color reproduction of a printing machine.
Within the context of the invention of the instant application, a printing machine can be regarded in very simplified form as including three components for each color to be printed, namely the printing form or plate, an ink source supplying the printing ink, and a transport device for transferring ink discharged by the ink source to the printing form or plate in the form of an ink layer.
During printing, individual printing inks are often distributed very non-uniformly in a subject to be printed. A consequence thereof is that the printing form or plate accepts a large quantity of ink from the transport device in some areas, while in other areas little or no ink is accepted. This can lead to a non-uniform distribution of the ink layer on the transport device, so that ink from the heavily inked areas of the transport device can possibly pass over to areas of the printing form or plate where it is not desired. The result may be a faulty, spotty printed image.
Metering of ink from the ink source to the transport device is generally performed zonally, i.e., zone by zone. Zones which correspond to areas in the printing image wherein the relevant color is represented only to a limited extent are supplied with a lesser quantity of ink than other zones.
In order to prevent boundaries between two zones from becoming visible in the printed image during the zonal metering, it is necessary to distribute the ink, which has been applied to the transport device, transversely to the printing direction on the path of the ink from the ink source to the printing form. The transverse distribution causes the quantity of ink which is transferred to the printing form in a zone defined by the transport device to be only to some extent the same as that which was applied by the ink source to the same zone of the transport device. Part of the ink originally applied into the zone has been displaced into adjacent zones by the distribution, and parts of ink layers originally applied to adjacent zones have been intermixed.
Moreover, inking-unit simulation programs have become known heretofore which permit the calculation of nominal or desired values for the ink metering, which should be complied with in order to achieve a good printed result, based upon a large number of variables, such as the type of printing ink used, the type of printing material, the moisture content, the sequence of colors during printing, and so forth. These programs describe the chronological development of the ink layer thicknesses in an inking unit in the course of a printing operation, starting from an initial distribution of the ink, metering thereof by the inking unit, and acceptance of ink by the printed material and, for this purpose, calculate step by step the effects of each movement of the inking unit on the ink distribution. With the aid of this model it is possible to calculate a set of metering variables to be adjusted at the ink source for the various printing inks for the purpose of ink presetting at the beginning of a printing job. Because these programs calculate the chronological development of the ink layer thicknesses numerically, and because the number of printing operations needed to achieve a steady state of the printing machine can run up to 1000 sheets or more, the computing effort associated with the use of these programs is enormous. Controlling production printing by using such programs is therefore not economically possible.
The published European Patent Document EP 0 228 347 B1, and the published German Patent Documents DE 195 33 822 A1 and DE 196 02 103 A1 disclose methods of controlling or regulating the color reproduction of a printing machine in production printing. In these methods, at selected points of a printed image, color values are measured and compared with corresponding values from an original. Depending upon the type of the established deviation, the metering of individual printing inks is varied in order to match the printed result to the desired or nominal value.
In this regard, the problem arises that, if a color deviation is registered in a given zone, and the ink metering for the relevant zone is correspondingly changed, this change influences a large number of other zones because of the ink exchange caused by the distribution. For example, eliminating a color deviation in one zone can readily lead to color errors then occurring in one or more other zones which previously supplied a printing result satisfactory in terms of color. A renewed correction of these color errors can, in, turn react on the zone considered first, and on further zones. There is thus the risk of the entire color regulation becoming unstable and the printed results becoming completely unusable and, even if ultimately metering variables are found which supply satisfactory color reproduction for the entire image to be printed, this is nevertheless preceded by a lengthy regulating process, in the course of which a great number of rejects have been produced. In addition, the extent of the correction needed to eliminate a given deviation depends upon settings of the transport system, such as lateral distribution and dampening. Each change in these settings of the ink transport system therefore necessitates renewed learning of the relationships between the extent of the error and the extent of the correction.
Both when determining presettings for a printing machine and during the continuous readjustment of the settings of the machine, the problem therefore arises that the settings and the color values obtained therewith in the printed result are interrelated in an extremely complicated manner.
When determining the presettings by simulation, the user can initially select only more-or-less arbitrarily setting values for which he or she causes the simulation to be performed, can estimate, based upon the simulation result, what setting or settings may possibly have to be changed in order to improve the color reproduction, and can repeat the simulation with accordingly changed settings. By performing a great number of simulations, it is then ultimately possible to find a set of presettings which promises satisfactory results; it is, however, not possible to assess whether this set is the best possible.
Even when regulating the settings during production printing, if a deviation from the desired color reproduction is determined, it is not directly possible for a correction to the settings to be specified which promises to correct only the determined deviation accurately and without any disruptive accompanying phenomena. Instead, it is possible only to feel one""s way to the desired or nominal color reproduction step by step by observing the effects of changes to the settings.
It is accordingly an object of the invention to provide a method of controlling an ink layer on a printing form of a printing machine which avoids the aforementioned disadvantages heretofore known in the prior art.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a method of controlling parameters of an ink layer at a selected location in a printing unit of a printing machine, the printing machine including at least one ink source for producing the ink layer on a transport device, whereon metering variables for regulating the application of ink to the transport device are zonally settable, and further including the transport device for transferring the ink layer to the selected location, the method comprising, for each zone of the ink layer, using a subject to be printed for determining desired values of parameters which the ink layer is to have at the selected location, and setting the metering variables of the ink source, based upon the desired values of the parameters, so that the parameters of the ink layer as the ink layer is applied to the transport device have temporary values deviating from the desired values, the deviation being such that an exchange of ink between the zones, taking place in the transport device, leads to the ink layer reaching the desired values of the parameters as the ink layer is transferred to a printing form.
In accordance with another mode, the method invention includes calculating the temporary values with the aid of a system of linear equations describing the parameters of the ink layer in the entire inking unit in a stationary state based upon the temporary values, taking into account ink splitting and lateral distribution.
In accordance with a further mode, the method invention includes solving the system of equations for the temporary values in order to calculate the temporary values for given desired values.
In accordance with an added mode, the method invention includes assuming that the ink splitting in the entire printing machine is half and half.
In accordance with an additional mode, the method invention includes applying the method for regulating production printing.
In accordance with yet another mode, the method invention includes applying the method for print presetting.
In accordance with yet a further mode, the method invention includes controlling the parameters at the selected location selected from the group thereof consisting of the printing material, a blanket cylinder and the printing form.
In accordance with yet an added mode, the method invention includes calculating the temporary values while taking into account at least one of the parameters of the ink layer consisting of the thickness and the dampening solution content thereof.
In accordance with a concomitant mode, the method invention includes using the averages of the degree of coverage of the printing inks for each zone for determining the temporary values.
The method invention of the instant application thus determines desired or nominal values of parameters of the ink layer which the ink layer is to have as it is transferred to the printing form or plate, by using the desired or nominal values to calculate so-called temporary values which the parameters must have when the ink layer is produced in order that the desired or nominal values be met at the time of transfer of the ink layer to the printing form or plate, and by setting the metering variables in order to produce the ink layer with the temporary values.
This method is suitable for determining presettings and also for regulating the settings during production printing.
In the first case, the determination of the desired or nominal values is based upon data from the printing original.
When the method is used for regulating the settings during production printing, a printed image of the subject is measured in order to determine the desired or nominal values. In the event of a deviation between the printed image and the original, the desired or nominal values are redetermined based upon the current values of the parameters and the determined deviation.
In order to be able to determine the temporary values, it is expedient to determine, for each zone, that percentage proportion of a quantity of ink applied in this zone by the ink source which is transferred onto the printing plate in this zone and in the other zones. These proportions depend upon the printing parameters (lateral distribution, moisture content) and the subject. The determination of this relationship can be performed empirically or computationally or by a combination of the two, for example, by computational interpolation of empirical data. It is obvious that such a computation requires significantly less time than the aforedescribed, previously conventional type of optimization which, for each optimization step, required the printing of at least one proof and the evaluation of the colors thereof.
A particularly rapid option for finding the suitable temporary values for a given set of desired values is to form a vector from the desired values and to multiply this vector by a square matrix. A suitable square matrix can be found in a straightforward manner by combining the determined proportions for the ink transfer between the various zones into a matrix, and by inverting this matrix.
Stated in more concrete terms, a required inking zone opening is determined by describing the stationary state of the inking unit as a system of equations. In this case, in order to simplify the mathematical description, half-and-half ink splitting is preferably assumed.
The ink layer thicknesses, respectively, in one zone on a pair of rollers is then described by
SDi=0.5(SDj+SDk), 
where SDj and SDk respectively refer to layer thicknesses on the rollers before a splitting point between these rollers, and SDi refers to the identical layer thicknesses on the two rollers downline from the splitting point.
In the case of an inking unit having n zones and m splitting points between ink source and printing material, it is possible to set up n(mxe2x88x921) equations of the above type. These equations correspond to nm unknowns. The system of equations therefore has m free parameters, for example, the ink layer thicknesses in the individual zones on the printing material, which can be defined. By solving the system of equations obtained in this manner, the layer thicknesses at all points in the inking unit and, in particular, on the first roller thereof before the passage through the first splitting point, i.e., the temporary values, can be determined.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as a method of controlling an ink layer on a printing form of a printing machine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein: